[flang] Better recovery from errors in a loop control

flang/docs/ParserCombinators.md

@@ -178,3 +178,16 @@ is built.  All of the following parsers consume characters acquired from
 Last, a string literal `"..."_debug` denotes a parser that emits the string to
 `llvm::errs` and succeeds.  It is useful for tracing while debugging a parser but should
 obviously not be committed for production code.
+
+### Messages
+A list of generated error and warning messages is maintained in the `ParseState`.
+The parser combinator that handles alternatives (`||` and `first()`) will
+discard the messages from alternatives that fail when there is an alternative
+that succeeds.
+But when no alternative succeeds, and the alternative parser as a whole is
+failing, the messages that survive are chosen from the alternative that
+recognized any input tokens, if only one alternative did so;
+and when multiple alternatives recognized tokens, the messages from the
+alternative that proceeded the furthest into the input are retained.
+This strategy tends to show the most useful error messages to the user
+in situations where a statement fails to parse.

flang/lib/Parser/executable-parsers.cpp

@@ -9,7 +9,6 @@
 // Per-type parsers for executable statements
 
 #include "basic-parsers.h"
-#include "debug-parser.h"
 #include "expr-parsers.h"
 #include "misc-parsers.h"
 #include "stmt-parser.h"
@@ -282,18 +281,26 @@ TYPE_CONTEXT_PARSER("loop control"_en_US,
                 "CONCURRENT" >> concurrentHeader,
                 many(Parser<LocalitySpec>{})))))
 
+// "DO" is a valid statement, so the loop control is optional; but for
+// better recovery from errors in the loop control, don't parse a
+// DO statement with a bad loop control as a DO statement that has
+// no loop control and is followed by garbage.
+static constexpr auto loopControlOrEndOfStmt{
+    construct<std::optional<LoopControl>>(Parser<LoopControl>{}) ||
+    lookAhead(";\n"_ch) >> construct<std::optional<LoopControl>>()};
+
 // R1121 label-do-stmt -> [do-construct-name :] DO label [loop-control]
 // A label-do-stmt with a do-construct-name is parsed as a nonlabel-do-stmt
 // with an optional label.
 TYPE_CONTEXT_PARSER("label DO statement"_en_US,
-    construct<LabelDoStmt>("DO" >> label, maybe(loopControl)))
+    construct<LabelDoStmt>("DO" >> label, loopControlOrEndOfStmt))
 
 // R1122 nonlabel-do-stmt -> [do-construct-name :] DO [loop-control]
 TYPE_CONTEXT_PARSER("nonlabel DO statement"_en_US,
     construct<NonLabelDoStmt>(
-        name / ":", "DO" >> maybe(label), maybe(loopControl)) ||
+        name / ":", "DO" >> maybe(label), loopControlOrEndOfStmt) ||
         construct<NonLabelDoStmt>(construct<std::optional<Name>>(),
-            construct<std::optional<Label>>(), "DO" >> maybe(loopControl)))
+            construct<std::optional<Label>>(), "DO" >> loopControlOrEndOfStmt))
 
 // R1132 end-do-stmt -> END DO [do-construct-name]
 TYPE_CONTEXT_PARSER("END DO statement"_en_US,

flang/lib/Parser/type-parsers.h

@@ -102,7 +102,6 @@ constexpr Parser<ForallAssignmentStmt> forallAssignmentStmt; // R1053
 constexpr Parser<ForallStmt> forallStmt; // R1055
 constexpr Parser<Selector> selector; // R1105
 constexpr Parser<EndSelectStmt> endSelectStmt; // R1143 & R1151 & R1155
-constexpr Parser<LoopControl> loopControl; // R1123
 constexpr Parser<ConcurrentHeader> concurrentHeader; // R1125
 constexpr Parser<IoUnit> ioUnit; // R1201, R1203
 constexpr Parser<FileUnitNumber> fileUnitNumber; // R1202

flang/test/Parser/recovery07.f90

@@ -0,0 +1,6 @@
+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! CHECK: error: expected ':'
+! CHECK: in the context: loop control
+do concurrent(I = 1, N)
+end do
+end